Whiplash and its associated syndromes continue to be ranked among the most common and debilitating nonfatal injuries. The facet joint has been identified as having the potential to undergo subfailure mechanical injury for motions the neck experiences during whiplash. Also, its neurophysiologic anatomy suggests its ability to generate pain for extreme loading. However, little is known about the effects of these local joint mechanics, such as tension and compression that are experienced in whiplash, on pain symptoms or signaling mechanisms. Purpose of Proposed Research: The lonq-term objective of this application is to define the mechanical conditions for dynamic facet joint loading that cause persistent pain symptoms, and to correlate behavioral outcomes with cellular reactivity in the spinal cord. Mechanical thresholds will provide the foundation to develop guidelines to reduce the incidence of painful whiplash injuries in the general population. They will also guide modifications to the automobile and occupant-use habits to reduce risks for these injuries. Methods: In the proposed research plan, there will be integration of biomechanics, behavioral test instruments and biochemical assays to link pain pathways with mechanical loading of the cervical facet joint. Using a new micromechanical device developed in this laboratory, the investigators will apply controlled distraction(s) and compression(s) of the facet joint and study the onset of a graded persistent pain response in the rat. It is hypothesized that: 1) dynamic facet joint distraction mimicking that observed in whiplash simulations will produce persistent behavioral hypersensitivity; 2) facet joint compression producing capsule pinch is sufficient to initiate persistent pain; and 3) for injury conditions above the threshold for producing pain, cellular reactivity responses of nociception and glial activation in the spinal cord will be produced and sustained. Implications for Prevention: By accomplishing the specific aims of the research, the investigators will directly link the initial mechanical conditions of the facet joint to pain pathways in the CNS. In turn, they will define the etiology for persistent pain due to dynamic joint motions, leading to the development and recommendations for future design interventions and behavioral modifications to prevent these injuries. Relevance: Whiplash is a tremendous public health problem and a common injury for vehicle occupants, with staggering annual societal and financial burdens. This research proposal will define the mechanical thresholds for painful facet injuries in the neck and investigate the effects of biomechanical risk factors, such as loading direction, on exacerbating pain. Physiologic correlates of these injuries and symptoms are also characterized for guiding future development of preventions and treatments for neck pain.